The invention relates to the field of fabricating layers of materials on a substrate. More specifically, it relates to a process for the chemical vapor deposition of materials on a substrate in order to make layers, possibly monocrystalline layers. The invention also relates to a reactor for implementing this process.
Many devices are already known for depositing layers on substrates by so-called CVD (Chemical Vapor Deposition) or MOCVD (Metal Organic Chemical Vapor Deposition, i.e. when one or more precursors are present in the form of organometallic compounds) methods.
To obtain layers of material with a quality high enough for the applications for which they are intended, it is necessary to heat the substrate on which they are deposited. Generally, the substrate is heated by thermal conduction between it and a substrate holder, sometimes also called a susceptor, which is itself heated by induction, by resistance heating or by radiation generated by lamps.
For example, PCT International Application WO 96/23912 teaches a reactor for the epitaxial growth of silicon carbide by a CVD method. This reactor comprises an external chamber and an internal duct. The carrier gases and the growth gases flow into the internal duct but can also communicate with the space lying between the internal duct and the external chamber. In such a reactor, the substrate on which the layers have to be deposited is placed in the gas stream, in the internal duct. This substrate is heated by heating means comprising a susceptor and a coil. The susceptor is placed beneath that region of the internal duct in which the substrate is located during the deposition. The susceptor is near the internal duct or in contact with it. The susceptor may even form part of the wall of the duct, in this region. The coil generates a radiofrequency field in the region of the susceptor so that the susceptor, when excited by this radiofrequency field, produces heat.
Also known, from the document Patent Abstract of Japan, Vol. 7, No. 251, is a reactor for vapor deposition of layers of a material on a substrate which extends mainly in a plane, comprising first and second heating means consisting of lamps and located outside a quartz duct, on either side of the substrate which is located in the duct.
However, on the one hand, the energy balance of this mode of heating, like that by radiation, is particularly poor because of poor coupling between generator (coils or lamps) and susceptor.
Sometimes the energy balance is worsened further when a quartz tube, in which the substrate holder is confined, is used for channeling the gases and when the tube is a double-walled tube in order to allow water circulation for cooling the tube. However, this further limits the coupling between the induction coil and the susceptor.
Thus, in order to deposit layers using this type of reactor, at temperatures which are sometimes greater than 1500xc2x0 C., it is necessary to supply it with a power often greater than 10 kilowatts, for a reactor with a capacity for a substrate 50 mm in diameter. In addition, the induction heating requires expensive investments because of this technology and of the necessary oversizing when high-frequency generators are used.
On the other hand, the energy balance may be just as poor, whatever the mode of heating, because of poor thermal coupling between the substrate holder and the substrate. This is because, when the substrate is simply placed on the substrate holder, the heat exchange takes place poorly, by solid conduction, very little, or even hardly at all, by gaseous conduction, most especially if the gas pressure is very low, and also very little by radiation, if the substrate is transparent in the radiation range of the susceptor. There are solutions for alleviating these problems, such as by adhesively bonding the substrate to the substrate holder or by depositing an -absorbent layer behind the substrate. However, these solutions require undesirable additional preparation steps; the materials for the adhesive bonding or those for being deposited on the rear face may contaminate the deposition reactors and the layers produced in these reactors; sometimes even, these solutions are ineffective for high-temperature heating.
One objective of the invention is to provide a process for depositing thin-film materials on a substrate, which is more economical, especially by virtue of a better energy balance than that allowed by the devices of the prior art.
This object is achieved by virtue of the invention which, in one of its aspects, is a process for the vapor deposition of layers of a material on a substrate which extends generally in a plane, characterized in that it comprises:
a step consisting in placing the substrate in a duct made of a refractory material and swept by the gaseous compounds necessary for the deposition, this duct being interposed between the substrate and first and second heating means located on either side of the plane of the substrate and
a step consisting in heating the substrate by virtue of the radiation from the heat of the duct, which is itself heated by the first and second heating means.
Thus, by virtue of the process according to the invention, the two main faces of a generally plane substrate are heated. The heating of the deposition face by the second means makes it possible to compensate for the radiative thermal losses from this face. Consequently, the temperature desired for this face is reached by heating the opposite face less, by the first heating means. In this way, it is possible to consume a smaller amount of power in the first heating means. Overall, this reduced power is not offset by the consumption of the second heating means. The deposition process according to the invention is therefore more economical than the deposition processes already known.
Furthermore, since the substrate is placed in a duct swept by the gaseous compounds necessary for the deposition, the duct being interposed between the substrate and the first and second heating means, the substrate is heated by the radiation from the heat of the duct, which is itself heated by the heating means located near the latter, but also by the gases which are themselves heated by the duct. This further improves the coupling between the heating means and the substrate. In addition, the duct channels the gas streams, thereby limiting any turbulence liable to disturb the growth of the layers of material on their substrate.
Advantageously, the process according to the invention comprises a step consisting in placing at least one heat shield around the first and second heating means.
Also advantageously, the process according to the invention comprises a step consisting in generating a temperature gradient perpendicular to the plane of the substrate and oriented in a first direction. The process according to the invention may even comprise a step consisting in reversing the direction of the temperature gradient with respect to the first direction. The fact of being able to choose and modify the direction of the gradient is a very advantageous option in the process according to the invention.
According to another aspect, the invention is a reactor for the vapor deposition of layers of a material on a substrate, which extends mainly in a plane, comprising first and second heating means located on either side of the plane of the substrate, characterized in that it furthermore comprises a duct, made of a refractory material and swept by the gaseous compounds necessary for the deposition, this duct being interposed between the substrate and first and second heating means. Advantageously, the reactor according to the invention comprises at least one heat shield around the first and second heating means.